I think that your embed is probably not the issue. The CSS looks OK, although you probably don't need it, you can just put the width in the iframe code as "100%". I think you just need to edit the content width of your section in the squarespace editor and change it to Large rather than Medium.
you may need the css for the padding as I think the embed will put in a padding of 17px by default, but that is consistent with other blocks, so try editing the content width first.
